As wide-angle lenses have been more and more widely used, especially with their use on aerial shooting apparatuses, camera distortion caused by the wide-angle lenses has attracted more and more attention. If correction is not made, images or videos taken by the wide-angle lenses may have a serious barrel distortion. For example, when a sport court is shot, straight lines sprayed on the court may be distorted and appear as curved lines. Therefore, a camera may need to be calibrated to obtain a distortion coefficient thereof to correct the images or videos taken by the camera.
Camera distortion generally includes radial distortion and tangential distortion. For a wide-angle lens, a fourth-order polynomial radial distortion model has been proved to be sufficient. Distortion equations of a wide-angle lens are:xd=xu(1+k1r2+k2r4),  (1)yd=yu(1+k1r2+k2r4),  (2)where (xu, yu) denote coordinates before distortion, also referred to as “non-distorted coordinates,” and (xu, yu) denote corresponding coordinates after distortion, also referred to as “distorted coordinates.” A distortion center can be represented by its coordinates (cx, cy), and the parameter r in Equations (1) and (2) can be calculated using r=√{square root over ((xu−cx)2+(yu−cy)2)}. The purpose of the calibration is to determine distortion coefficients k=(k1, k2) and the distortion center c=(cx, cy).
Usually, the distortion center (cx, cy) in an image is close to a central point
  (            w      2        ,          h      2        )of the image, where w and h are the width and the height of the image, respectively. The central point of the image can be used as an initial value of the distortion center, and an optimal solution to the distortion center can be obtained by a small number of times of iteration. However, it may be relatively more difficult to calculate the distortion coefficients.
In conventional technologies, a lens can be calibrated using a calibration board. In particular, a series of images or videos of the calibration board can be taken, and then internal references and distortion coefficients can be calculated according to geometric constraints. After the distortion coefficients are obtained, the shot videos or images can be corrected. Calibration using the calibration board can be relatively accurate as long as a high-accuracy calibration board is used. This is a barrier to ordinary users when performing calibration.